When small holes are formed in a metal or alloy workpiece by means of electrical discharge machining, an electrode is used which is formed of a copper tube having an outer diameter for example in the range from 0.3 to 3.0 mm. A working fluid for cooling is passed through through holes of the electrode tube, electrical discharge is caused between the electrode tube and the workpiece while turning the electrode tube, whereby the workpiece is perforated.
However, as illustrated in a cross section shown in FIG. 8, an undesired core 7 forms inside an electrode tube 1 during electrical discharge machining of a workpiece 6. The core 7 lowers the precision of the electrical discharge machining, impedes a smooth electrical discharge machining operation and lowers the working efficiency as a result.
In order to solve such a problem, electrode tubes shown in FIGS. 9 to 11 have been suggested (see Japanese Patent Laying-Open No. 63-11239) (Watanabe et al., published Jan. 18, 1988).
An electrode tube 1 in FIG. 9 includes a cylindrical metal tube 2 and a metal partition 8 inserted in the cylindrical tube 2.
An electrode tube 1 illustrated in FIG. 10 includes a cylindrical metal tube 2 and two metal rods 9 inserted in cylindrical tube 2.
An electrode tube 1 shown in FIG. 11 includes a cylindrical metal tube 2 and two inner metal tubes 10 inserted in cylindrical tube 2.
The space in the cylindrical tube 2 of the electrode 1 shown in FIGS. 9 to 11 is divided into a plurality of through holes. Therefore, if electrical discharge machining is conducted while turning the electrode 1, an undesired core does not form in the electrode tube 1.
In the manufacture of electrode tubes as illustrated in FIGS. 9 to 11, a partitioning plate, thin rods, or inner tubes are inserted in tube 2 and then drawing operation must be performed. A tube material wound in a long coil cannot be used if such a partitioning plate, thin rods or thin inner tubes are inserted in the tube 2. The material of the tube 2 into which the partitioning plate, thin rods or thin inner tubes are inserted must be relatively short and straight. Therefore, the electrode tubes as shown in FIGS. 9 to 11 cannot be continuously produced from long tube material 2 initially wound in a coil for example, and the working or production efficiency cannot be improved.